Electron beam lens for micro-column electron beam apparatus and method of fabricating the same

ABSTRACT

Provided is an electron beam lens for a micro-column electron beam apparatus and a method of manufacturing the same. A photosensitive glass substrate is used as a base isolation substrate and a thin metal film is grown by a plating method. Holes through which electron beam passes are formed by a lift off method after forming a resist pattern shaped as a hole on a seed metal layer and plating the thin metal film.

The present patent application is a Divisional of application Ser. No.10/817,779, filed Apr. 2, 2004 now U.S. Pat. No. 6,996,896.

BACKGROUND OF THE INVENTION

This application claims the priority of Korean Patent Application No.2003-74927 filed on Oct. 25, 2003, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein in its entiretyby reference.

1. Field of the Invention

The present invention relates to an electron beam lens of a micro-columnelectron beam apparatus used for an electron beam exposure process of asemiconductor manufacturing process and a method of fabricating thesame.

2. Description of the Related Art

A micro-column electron beam apparatus is used for patterning asemiconductor device as designed by focusing an electron beam generatedat a cathode onto a resist film coated on a semiconductor wafer or amask. The micro-column electron beam apparatus includes a deflector andelectron beam lenses such as a source lens and a focus lens.

A deflector disposed between the electron beam lenses of themicro-column electron beam apparatus electrically controls the directionof the electron beam generated at the cathode such that the electronbeam is projected onto the wafer or mask according to design datareceived from a pattern generator.

An electron beam lens of the micro-column electron beam apparatusincludes a plurality of sequentially stacked thin conductive films inwhich a base isolation substrate is disposed between the conductivefilms. A hole through which the electron beam passes is formed at acenter of each of the conductive thin films. Each of the base isolationsubstrates that acts as an insulator to maintain a uniform distancebetween adjacent conductive thin films, has a hollowed space which islarger than the hole through which the electron beam passes. Theelectron beam passing through is focused or dispersed by an electricpotential applied to the conductive thin films as it passes through theholes in the central portion of the conductive thin films.

In a conventional method of manufacturing the electron beam lens, theconductive thin films are manufactured separately with a thickness ofseveral μm and attached to both sides of an individual substrate such aspyrex glass using an anodic bonding method. The conductive thin film isa conductive silicon thin film including a boron doped layer on a waferor a thin metal sheet of chrome, wherein the conductive silicon thinfilm is formed by forming a boron doped layer on a silicon wafer thathas a thickness of 0.2˜1 mm, forming a membrane on by etching backsideof the silicon wafer, and then forming a hole by reactive ion etchingprocess that leaves the boron doped layer on the wafer.

In order to manufacture an electron beam lens capable of focusing ordispersing the electron beams as intended, the conductive thin filmsmust be parallel and the holes must be positioned accurately withrespect to the central portion during a stacking process of theconductive thin films. However, in the prior art, parallel arrangementof the conductive thin films and the in-line alignment of the holes isvery difficult because each conductive film is separately manufacturedand stacked. Particularly, a required bonding technique is verycomplicated and troublesome. Therefore, the conventional method ofmanufacturing an electron beam lens has poor reproducibility andproduces lenses with a weak mechanical structure.

SUMMARY OF THE INVENTION

The present invention provides an electron beam lens which provides asuperior performance since thin conductive films are arranged inparallel, holes are correctly self-aligned, and holes have uniformshapes.

The present invention also provides a method of manufacturing anelectron beam lens in which conductive thin films having holes that areself aligned.

According to an aspect of the present invention, there is an electronbeam lens of a micro-column electron beam apparatus comprising aplurality of conductive thin films, each having a hole through whichelectron beam passes; and a plurality of base isolation substratesdisposed between the conductive thin films, each of the base isolationsubstrate having hollowed spaces larger than the holes, wherein the baseisolation substrates are photosensitive glass substrates in which thehollowed spaces are formed by etching activated regions formed byexposing to light, the conductive thin films are metal plating layersgrown on the base isolation substrates, the photosensitive glass, andthe holes are each formed by forming a resist pattern on the baseisolation substrate along a central line of an electron beam path,growing the metal plating layer on the base isolation substrate, andthen performing a lift-off method to remove the resist pattern, so thatthe holes are self aligned along the central line of an electron beampath.

According to another aspect of the present invention, there is a methodof manufacturing an electron beam lens comprising focusing a light ontoa region with a shape of a hollowed space to form an activated region ina central portion of a photosensitive glass substrate; forming a seedmetal layer on at least one surface of the photosensitive glasssubstrate; forming a resist pattern with a shape of a hole on thecentral portion of the seed metal layer; forming a plating layer on aregion of the seed metal layer exposed resist pattern; removing theresist pattern using a lift off process; forming a thin metal filmhaving a hole on the photosensitive glass substrate by removing aportion of the seed metal layer that has covered by the resist pattern;and forming a hollowed space by etching the activated region of thephotosensitive glass substrate.

In the method of manufacturing the electron beam lens according to thepresent invention, a plurality of the photosensitive glass substratescan be bonded together after forming a thin metal film having a hole oneach of the photosensitive glass substrates. The activated portions ofthe photosensitive glass substrate can then be removed simultaneously orone by one to form hollowed spaces between the plurality of conductivethin metal films. In this manner, an electron beam lens or an electronbeam module structure with a stack of photosensitive glass substratesbetween thin metal films can be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail preferred embodimentsthereof with reference to the attached drawings in which:

FIGS. 1 through 12 are cross-sectional views illustrating a method ofmanufacturing an electron beam lens, according to a first exemplaryembodiment of the present invention;

FIG. 13 is a cross-sectional view of a modified electron beam lensaccording to the present invention;

FIG. 14 is a bottom plane view of the electron beam lens illustrated inFIG. 13;

FIG. 15 is top-plane view of the electron beam lens illustrated in FIG.13;

FIG. 16 is a cross-sectional view of an electron beam lens according toa second embodiment of the present invention; and

FIG. 17 is a cross-sectional view of an electron beam lens according toa third exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, exemplary embodiments of the present invention will bedescribed more fully with reference to the accompanying drawings. Thisinvention may, however, be embodied in many different forms and shouldnot be construed as being limited to the embodiments set forth herein.Rather, these embodiments are described herein so that this disclosurewill be thorough, complete, and fully convey the concept of theinvention to those skilled in the art. In the drawings, the thicknessesof films and regions may be exaggerated for clarity. To facilitateunderstanding, identical reference numerals have been used, wherepossible, to designate identical elements that are common to thefigures.

In an exemplary embodiment of the present invention, a method ofmanufacturing an electron beam lens with a structure including threethin metal films and two base isolation substrates, which is arepresentative structure of a source lens and a focus lens, will bedescribed. However, it is understood that the method of forming anelectron beam lens according to the present invention is applicable tomanufacturing stacking structures with more or less thin metal films andbase isolation substrates.

FIGS. 1 through 12 are cross-sectional views illustrating a method ofmanufacturing an electron beam lens according to a first exemplaryembodiment of the present invention.

A squared photosensitive substrate having a thickness of 100˜500 μm isused for a base isolation substrate. Referring to FIG. 1, aphotosensitive mask 20 is formed on a central portion of a firstphotosensitive glass substrate 10, upper and lower surfaces of which areplanarized. The photosensitive mask 20 is formed on the firstphotosensitive glass substrate 10 with an opening O that has a circularshape with a diameter of 2 to 3 mm or a square shape with a side lengthof 2 to 3 mm, to form a first hollowed space. The first photosensitiveglass substrate 10 covered by the photosensitive mask 20 is exposed toultraviolet light 23.

FIG. 2 illustrates the structure after the photosensitive mask 20 isremoved and an activation treatment is performed. Referring to FIG. 2,an activated portion 25 as a circular shape with a diameter of 2 to 3 mmor a square shape with a side length of 2 to 3 mm is formed on a centralportion of the first photosensitive glass substrate 10. The shape of anactivated portion 25 may vary according to the shape of the opening Oand the method of exposure. For example, the shape of the activatedportion can be a cylinder, a square column, or a head-cut pyramid orcone. The activated portion 25 can be readily defined by controlling theshape of the opening O and the method of exposure since the substrate isformed of photosensitive glass.

Referring to FIG. 3, a first and a second seed metal layer 30 and 35required for metal plating are formed on the upper and lower surfaces ofthe photosensitive glass substrate 10, respectively. The first and thesecond seed metal layers 30 and 35 increase a uniformity of plating andacts as initial nucleation sites. The thicknesses of the first and thesecond seed metal layers 30 and 35 can be chosen appropriately. Adeposition may be performed by a sputtering method, but a CVD method canalso be used. The seed metal can be one of a metal selected from a groupconsisting of Cu, Au, Ag, Pt or Pd, but can also be selected accordingto a metal used for plating or a method of plating used.

Referring to FIG. 4, a resist film is coated on the first seed metallayer 30, and exposed to a light using a mask (not shown) with a shapeof a first hole. Then, the resist film, except a resist pattern 45 withthe shape of the first hole, is removed using a photolithography processthat develops and bakes the resist film, leaving the first seed metallayer 30 partially exposed. In the same manner, a resist film is coatedon the second seed metal layer 35, and exposed to a light by a doubleside exposure apparatus using a mask (not shown) with a shape of asecond hole. Then, the resist film, except the resist pattern 45 with ashape of the second hole, is removed using a photolithography process,leaving the second seed metal layer 35 partially exposed.

The resist pattern 45 with the shape of the second hole is formed byaligning with the resist pattern 40 with the shape of the first hole.The resist patterns 40 and 45 have a circular shape with a diameter of0.5 to 200 μm, and define a hole through which electron beam passes. Thedouble side exposure apparatus is used for forming the resist pattern 45with the shape of the second hole on the lower surface of the substrate,and the pattern 45 is aligned to the pattern 40 with the shape of thefirst hole.

Referring to FIG. 5, the resultant structure is plated. The plating canbe performed by an electroplating method or an electroless platingmethod. The electroplating method has a high plating efficiency and arate of plating can be controlled easily, but is a complex process andrequires a complex apparatus. On the other hand, the electroless platingmethod has a higher filling characteristic than the electroplatingmethod, but has a low plating efficiency and it is difficult to controlrate of plating. The electroplating method is more suitable for theexemplary embodiment of the present invention, but not limited thereto.

The plating process can be performed as follows. A first metal platinglayer 50 is formed on the first seed metal layer 30 on which the resistpattern 40 with the shape of the first hole is formed, and a secondmetal plating layer 55 is formed on the second seed metal layer 35 onwhich the resist pattern 45 with the shape of the second hole is formed.The first and the second metal plating layers 50 and 55 are grown on theportions of the surfaces of the first and the second seed metal layers30 and 35 exposed by the resist patterns 40 and 45 with the shapes ofthe first and the second holes. A thickness of the plating can be set asrequired, but it is preferably 2 to 35 μm. The plating layers can bemade of copper, a copper alloy, stainless steel, nickel, gold, silver,or platinum. However, it is understood that these metals are exemplarymaterials and that all metals that are consistent with the purpose ofthe present invention can be used. Plating can be performed using aconventional method. For example, to plate copper using theelectroplating method, a plating solution can be an aqueous solutionthat provides 2⁺ copper ions such as an aqueous copper sulfuric acidsolution (CuSO₄.5H₂O), sulfuric acid (H₂SO₄) solution, or hydrochloricacid (HCl) solution.

Referring to FIG. 6, a lift off process for removing the resist patterns40 and 45 with the shapes of the first and the second holes isperformed. The lift off process is performed by a wet method that usesan organic solvent to remove a resist material. Next, portions of theseed metal layers 30 and 35 covered by the resist patterns 40 and 45with the shapes of the first and the second holes are removed. Thus, afirst metal thin film 50′ having a first hole 60 on the upper surface ofthe photosensitive glass substrate 10 and a second metal thin film 55′having a second hole 65 on the lower surface of the photosensitive glasssubstrate 10 are formed. The first and the second holes 60 and 65 areformed in the same location as the resist patterns 40 and 45 are selfaligned by the double side exposure apparatus. Accordingly, an electronbeam lens capable of controlling the focusing or dispersion of theelectron beam can be manufactured. Since the first and second holes 60and 65 are respectively formed in the same locations as the resistpatterns 40 and 45 are formed, a hole with a correct concentricity andeccentricity and well defined shape is formed compared to a hole formedby a conventional method in which the hole is perforated after formingthe individual conductive thin films. Accordingly, a superior quality ofelectron beam lens can be manufactured.

Referring to FIG. 7, a second photosensitive glass substrate 110 thathas planarized upper and lower surfaces is prepared. The secondphotosensitive glass substrate 110 can be prepared as a similar methodas described with reference to FIGS. 1 through 6. First, activatedportion 125 with a shape of a second hollowed space is prepared byexposing a central portion of the second photosensitive glass substrate110 to UV light. The activated portion 125 with the shape of the secondhollowed space can be a circle with a diameter of 2 to 3 mm or a squarewith a side line of 2 to 3 mm, however, it can be formed to be the samesize and/or shape as the activated portion 25, or it can be formed to bebigger or smaller than the activated portion 25. Next, a thin metalplating layer 130 and a third seed metal layer 135 are formed on theupper and lower surfaces of the second photosensitive glass substrate110, respectively. As illustrated in FIG. 7, the thin metal platinglayer 130 can be formed on a seed metal layer 129. Alternatively, thethin metal plating layer 130 can be the seed metal layer itself. Thethin metal plating layer 130 can be formed of a metal selected from agroup consisting of copper, a copper alloy, stainless steel, nickel,gold, silver, or platinum. The thin metal plating layer 130 exposes theactivated portion 125.

Referring to FIG. 8, the structures of FIG. 6 and the FIG. 7 arecombined. That is, the second thin metal film 55′ on the lower surfaceof the first photosensitive glass substrate 10 and the thin metalplating layer 130 on the upper surface of the second photosensitiveglass substrate 110 are bonded using a eutectic bonding method. Then,the second thin metal film 55′, and the thin metal plating layer 130constitute a thin metal film 131. A bonding procedure used can bealtered as necessary. For example, the second photosensitive glasssubstrate 110 can be bonded to the second thin metal film 55′ on thefirst photosensitive glass substrate 10 using a bonding material such asa glass paste, instead of bonding thin metal plating layer 130 to thesecond photosensitive glass substrate 110.

Next, referring to FIG. 9, a resist pattern 145 with a shape of a thirdhole that is aligned to the first and second holes 60 and 65 is formedon the third seed metal layer 135. A surface on which the first thinmetal film 50′, the first hole 60, and the activated region 25 areformed, is protected by a resist film 150, and the resist pattern 145with the shape of the third hole is formed on the opposite surface onwhich the third seed metal layer 135 is formed, by a photolithographyprocess using a predetermined mask. A diameter of the resist pattern 145can be 0.5 μm to a few 100 μm. Preferably, a double side exposureapparatus for aligning is used. The double side exposure apparatus formsthe resist pattern 145 with the shape of the third hole aligned to thefirst hole 60 by the double side exposure apparatus.

Referring to FIG. 10, a third metal plating layer 155 is formed on aportion of the third seed metal layer 135 exposed by the resist pattern145. The third metal plating layer 155 can be formed of a metal selectedfrom a group consisting of copper, a copper alloy, stainless steel,nickel, gold, silver, or platinum.

Referring to FIG. 11, the resist pattern 145 is removed by a lift offprocess. At this time, the resist film 150 is also removed. A third thinmetal film 155′ having a third hole 165 is formed by removing a portionof the seed metal layer covered by the resist pattern 145 with the shapeof the third hole 165 from the lower surface of the secondphotosensitive glass substrate 110. The third hole 165 is formed at thesame location as the resist pattern 145, which aligned with the firsthole by the double side exposure apparatus.

Referring to FIG. 12, a first and a second hollowed space 70 and 175 areformed by wet etching the activated portion 25 of the firstphotosensitive glass substrate 10 and the activated portion 125 of thesecond photosensitive glass substrate 110. The activated portion can beremoved by, for example, a wet etching method using a fluoric acidsolution.

According to the foregoing exemplary embodiment, an electron beam lenshaving three of thin metal film layers, i.e., the first thin metal film50′, the thin metal film 131 in the bonding region, and the third thinmetal film 155′, and two of base isolation substrates, i.e., the firstand the second photosensitive glass substrates 10 and 110 can bemanufactured. Referring to FIG. 12, the holes 60, 65, and 165 throughwhich an electron beam passes, located in the central portion of theelectron beam lens are self aligned. The thin metal films, i.e., thefirst thin metal film 50′, the thin metal film 131 at the bondingregion, and the third thin metal film 155′ are need not be bonded to thebase isolation substrates as in the conventional method because the thinmetal films are directly grown on the first or the second photosensitivesubstrate 10 and 110. Therefore, troublesome work for accuratelypositioning the holes of the thin metal films and the substrates withrespect to the center of axis of electron beam is not required.Therefore, the easy manufacturing due to easy aligning results inimproved productivity and a superior quality of lens.

The main features of the present invention are that a photosensitiveglass substrate is used for the base isolation substrate, the thin metalfilm is grown by a plating process, and the holes through which electronbeam passes are aligned by the double side exposure techniques.

In the above exemplary embodiment, the first and the second hollowedspaces 70 and 175 are formed simultaneously by a single wet etchingprocess, but the first hollowed space 70 can be formed immediately afterperforming the steps illustrated in FIGS. 1 through 6. That is, thehollowed spaces can be formed simultaneously or one by one.

In the first exemplary embodiment of the present invention, the thinmetal films are formed on both the upper and lower surfaces of thephotosensitive glass substrate 10. However, the formation of the thinmetal film can be formed on a single surface of the photosensitive glasssubstrate using the metal plating and the lift off method.

FIG. 13 is a cross-sectional view of an electron beam lens with a thinmetal film only a face of the photosensitive glass substrate. Referringto FIG. 13, a thin metal film 50″ having a hole 60′ can be formed on aphotosensitive glass substrate 10′ having a hollowed space 70′. FIG. 14is a bottom plane view of an electron beam lens of FIG. 13 taken fromthe photosensitive glass substrate 10′ and FIG. 15 is a top plane viewof the electron beam lens of FIG. 13.

As described above, the hollowed space formed by exposing thephotosensitive glass substrate to a light can have a variety of shapesaccording to the shape of the opening of the photosensitive mask 20 inFIG. 1 and the method of exposing. The hollowed space depicted in FIGS.13 and 14 has a head-cut pyramid shape.

According to the second and third exemplary embodiments of the presentinvention, electron beam lenses as illustrated in FIGS. 16 and 17 can bemanufactured. The FIGS. 16 and 17 are examples of a source lens and afocus lens, respectively.

Referring to FIG. 16, a plurality of thin conductive films 250 a, 250 b,and 250 c respectively having holes 260 a, 260 b, and 260 c throughwhich electron beam passes, are disposed in the central region of thesource lens, and are disposed between the photosensitive glasssubstrates 210 a, 210 b, 210 c, and 210 d having hollowed spaces 225 a,225 b, 225 c, and 225 d which have large areas than the area of theholes 260 a, 260 b, and 260 c. The hollowed spaces 225 a, 225 b, 225 c,and 225 d are formed by etching activated portions formed by exposingthe photosensitive glass substrates 210 a, 210 b, 210 c, and 210 d to alight. The first and fourth hollowed spaces 225 a and 225 d have ahead-cut pyramid shape and the second and third hollowed spaces 225 band 225 c have a cylindrical shape or a square column shape. Theconductive thin films 250 a, 250 b, and 250 c are plating layers grownon the photosensitive glass substrates 225 a, 225 b, 225 c, and 225 d.The center of the holes 260 a, 260 b, and 260 c are self aligned along acenter axis of electron beam.

Referring to FIG. 17, a plurality of thin conductive films 350 a, 350 b,and 350 c respectively having holes 360 a, 360 b, and 360 c throughwhich electron beam passes, are disposed on the central region of thefocus lens, and are disposed between the photosensitive glass substrates310 a, 310 b, 310 c, and 310 d having hollowed spaces 325 a, 325 b, 325c, and 325 d which have larger areas than the area of the holes 360 a,360 b, and 360 c. The hollowed spaces 325 a, 325 b, 325 c, and 325 d areformed by etching activated portions formed by exposing thephotosensitive glass substrates 310 a, 310 b, 310 c, and 310 d to alight. The second hollowed space 325 b has a cylindrical shape or asquare column shape and the first, third, and fourth hollowed spaces 325a, 325 b and 325 d have a head-cut pyramid shape. The conductive thinfilms 350 a, 350 b, and 350 c are plating layers grown on thephotosensitive glass substrates 325 a, 325 b, 325 c, and 325 d. Thecenter of the holes 360 a, 360 b, and 360 c are self aligned along acenter axis of electron beam.

The electron beam lenses illustrated in FIGS. 16 and 17 can bemanufactured by further comprising protective films on both outer facessimilar to the first and the third thin metal films 50′ and 155′exposing the holes 60 and 165 in FIG. 12. Alternatively, the electronbeam lenses illustrated in FIGS. 16 and 17 can be formed by stackingstructures having a configuration as illustrated in FIG. 13.

According to the exemplary embodiment of the present invention, a singlebody electron beam lens in which holes formed in a plurality of thinmetal films can be correctly aligned, and the thin metal films and aplurality of base isolation substrates are formed in a single body thatdoes not require an additional bonding process, can be manufactured.

The electron beam according to the present invention is highlyreproducible and provides structural safety because it is manufacturedas a single body with correctly self aligned holes, thereby having ahigh durability. Since the electron beam lens can be manufactured usingthe plurality of thin films and base isolation substrates manufacturedby repeatedly performing the same method, the electron beam lensaccording to the present invention enables to be able to manufacture amodule of source lens, a focus lens, and further a single body electronbeam lens.

The method of manufacturing the electron beam lens according to theexemplary embodiment of the present invention provides simplicity in amanufacturing process and a low manufacturing cost because aphotosensitive glass substrate is used for a base isolation substratematerial, a thin metal film is formed by a plating method that has ahigher productivity, and a double side exposure apparatus is used toalign centers of the thin films. The electron beam lens according to theexemplary embodiment of the present invention has a superior performancesince the eccentricity and concentricity of the holes can be adequatelyformed, and because of efficiency in aligning and simplicity ofmanufacturing, productivity is increased.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the sprit and scopeof the invention as defined by the appended claims.

1. A method of manufacturing an electron beam lens comprising: focusinga light onto a region with a shape of a first hollowed space to form anactivated region on a central portion of a first photosensitive glasssubstrate; forming a first seed metal layer on a first surface of thefirst photosensitive glass substrate and forming a second seed metallayer on a second surface of the first photosensitive glass substrate;forming a first resist pattern with a shape of a first hole on the firstseed metal layer; forming a second resist pattern with a shape of asecond hole that is aligned with the first resist pattern on the secondseed metal layer; forming a first metal plating layer on a portion ofthe first seed metal layer exposed by the first resist pattern; forminga second metal plating layer on a portion of the second seed metal layerexposed by the second resist pattern; removing the first and the secondresist patterns using a lift off method; forming first and second thinmetal films having the first hole and the second hole, respectively, byremoving portions of the seed metal layers that were covered by thefirst and second resist patterns; forming a first hollowed space byetching the activated region of the first photosensitive glasssubstrate; focusing a light onto a region with a shape of a secondhollowed space to form an activated region of a central portion of asecond photosensitive glass substrate; forming a third thin metalplating layer that exposes a second hollowed space on first surface ofthe second photosensitive glass substrate and forming a third seed metallayer on a second surface of the second photosensitive glass substrate;bonding the second thin metal film of the first photosensitive glasssubstrate and the third thin metal film of the second photosensitiveglass substrate using a eutectic bonding method; forming a third resistpattern with a shape of a third hole that is aligned with the first andsecond resist patterns on the third seed metal layer; forming a thirdmetal plating layer on a portion of the third seed metal layer exposedby the third resist pattern; removing the third resist pattern using alift off method; forming a third thin metal film having the third holeby removing a portion of the third seed metal layer that is covered bythe third resist pattern; and forming a second hollowed space by etchingthe activated region of the second photosensitive glass substrate. 2.The method of claim 1, wherein the metal layer is formed of a metalselected from a group consisting of copper, a copper alloy, stainlesssteel, nickel, gold, silver, and platinum.
 3. The method of claim 1,wherein the second resist pattern with the shape of the second hole thatis aligned with the first resist pattern, and third resist pattern witha shape of the third hole that is aligned with the first and the secondhole are formed by using a double side exposure apparatus.
 4. The methodof claim 1 further comprising forming a protective film that exposes theholes on both outer surfaces of the thin metal film.
 5. The method ofclaim 1, the first hollowed space and the second hollowed space areformed at the same time.